Electrical generating facilities are normally interconnected with other similar facilities. This occurs for many reasons, among which are the necessity for back-up power supply in the event of loss of a unit, the need for shut-down of units to accomplish maintenance, varying loads, and the need for generating power most economically. In order to effect the required coordination between generating units, interconnecting transmission lines and users, it is necessary that there be effective and reliable communication between them. This communication has previously been effected in many ways well known to those skilled in the art. For example, there have been supervisory circuits dedicated to this purpose, e.g., telephone lines or carrier current circuits. However, separate dedicated circuits, especially those with the required degree of redundancy or other form of reliability, involve significant additional expense, and there therefore have been various proposals to provide the desired degree of communications effectiveness through signaling accomplished over the interconnecting transmission lines.
One major obstacle to the use of the transmission lines themselves has been the incredibly high noise level that exists on such lines. The extraordinarily high voltage on such lines induces some corona discharge despite design of system components to minimize it. Additionally, there is a small amount of leakage across supporting or other insulators, all of which tends to produce electrical noise. But of even greater significance are the spikes and other voltage disturbances occasioned by switching and lightning transients.
In order to overcome system noise while achieving the potential economies of using the transmission lines, there have previously been proposed methods of signaling which involve producing slight changes of frequency within an interconnection. Such frequency changes have signaled the desired information by one or a combination of: (1) absolute frequency change; (2) the rate of change of the frequency; or (3) the duration of a particular frequency. Such proposals are embodied in two United States Patents: U.S. Pat. No. 4,246,492 granted to John M. Vandling, on Jan. 20, 1981; and U.S. Pat. No. 4,370,563 granted to John M. Vandling, deceased (Patricia Vandling, executrix) on Jan. 25, 1983. However, the proposals of these patents were dependent upon knowledge of the absolute value of the system frequency. In addition, they required complex controllers to invoke the required frequency, rate-of-change of frequency, and duration. Moreover, since in large interconnected grids, extremely large quantities of power are generally required to effect the changes needed for signaling according to Vandling's proposals, the proposals of his patents have not found practical application. Accordingly, the need has continued for proposals that achieve the desired signaling characteristics while preserving reliability, economy and security.